C++ :

#include<iostream>
#include<algorithm>
#include<cstring>
#include<ctime>
#include<cmath>
#include<string>
#include<cstdio>
#include<stack>
using namespace std;

char ans[100], tmp[100], c[100];
char s[100];
char t1[100], t3[100];

void mul(char *a, char *b)
{

    int len1 = strlen(a);
    int len2 = strlen(b);
    int d[100];
    //cout<<a<<"***"<<b<<endl;
    memset(d, 0, sizeof(d));
    for(int i = 0; i < len1; ++i)
    {
        for(int j = 0; j < len2; ++j)
        {
            d[i+j] += (a[i] - '0') * (b[j] - '0');
        }
    }
    int t = 0, len = len1 + len2 - 1;
    for(int i = len - 1; i >= 0; --i)
    {
        d[i] += t;
        t = d[i] / 10;
        d[i] = d[i] % 10;
    }
    stack<int> st;
    while(t > 0)
    {
        st.push(t%10);
        t /= 10;
    }
    int x = 0;
    while(!st.empty())
    {
        c[x++] = char(st.top() + '0');
        st.pop();
    }
    for(int i = 0; i < len; ++i)
    {
        c[x++] = char(d[i] + '0');
    }
    c[x] = '\0';
    int i = 0;
    while(c[i] == '0') ++i;
    strcpy(c, c+i);
    if(strlen(c) == 0) c[0] = '0', c[1] = '\0';
    //cout<<c<<"<------->"<<endl;
}

bool Big(char *a, char *b)
{
    int len1 = strlen(a);
    int len2 = strlen(b);

    if(len1 > len2) return true;
    else if(len1 < len2) return false;
    else
    {
        for(int i = 0; i < len1; ++i)
        {
            if(a[i] != b[i])
                return a[i] > b[i] ? 1 : 0;
        }
    }
    return false;
}

void dfs(int l, int r, int k)
{
    //cout<<"dfs("<<l<<","<<r<<","<<k<<")"<<endl;
    if(k >= r - l) return ;
    if(k == 0)
    {
        int j, x = 0;
        for(j = l; j < r; ++j) t1[x++] = s[j];
        t1[x] = '\0';
        mul(tmp, t1);
        if(Big(c, ans))
            strcpy(ans, c);
        return;
    }
    for(int i = l; i < r; ++i)
    {
        int j, x = 0;
        for(j = l; j <= i; ++j) t1[x++] = s[j];
        t1[x] = '\0';
        char t2[100];
        strcpy(t2, tmp);
        mul(tmp, t1);
        strcpy(tmp, c);
        dfs(i+1, r, k-1);
        strcpy(tmp, t2);
    }
}

int main()
{
    //mul("2", "345");
    int n, k;
    cin>>n>>k>>s;
    ans[0] = '\0';
    tmp[0] = '1'; tmp[1] = '\0';
    dfs(0, n, k);
    cout<<ans<<endl;
    return 0;
}